1. Field of the Invention
A tool for mapping the texture and composition of the formation that comprises the sidewall of a borehole. The tool combines the attributes of a resistivity-type borehole imager with the benefits of a circumferential sidewall acoustic imager in a single sonde.
2. Discussion of Related Art
After an oil or gas borehole has been drilled into the earth, it is of interest to the geologist to study the texture and composition of the formations that make up the borehole sidewall. The term "texture" includes, but is not limited to, visible surface features such as cracks, faults, dip of the strata, rugosity, vugs and macro-crystalline structure, as well as the physical and chemical composition of the rock. Ideally, if the borehole fluid were clear water or air, one could lower a television camera into the hole and circumferentially scan the sidewall as the camera is raised back to the surface, thereby mapping visible features of the sidewall in two dimensions. Unfortunately, the drilling mud used during the drilling of the borehole is opaque to optical illumination. Techniques for imaging the sidewall texture in the presence of opaque drilling mud include, among others, electrical and acoustic methods, each of which has its own advantages.
For convenience in the ensuing discussion, but not to be considered in any way limiting, a distinction will be made between the terms "logging" and "imaging" or "imagery". A logging tool is designed to define the gross lithologic boundaries penetrated axially by the borehole without particular indication of circumferential variations. Usually the instrumentation used in a logging tool can penetrate relatively deeply into the formations that make up the borehole sidewall. Imaging tools, on the other hand, are intended to furnish a picture of the texture of the exposed sidewall surface with little or no penetration into the formation.
Electrical resistivity methods are useful in the presence of conductive drilling muds and particularly in circumstances where the different formation rock types have the same acoustic impedance. In the presence of highly resistive rocks or drilling fluids, spontaneous potential (SP) measurements may be employed. Acoustic methods are preferred with highly resistive oil base muds or where there is a low conductivity contrast between two formations. Acoustic illumination is capable of detecting micro-cracks due to drilling stresses imparted to the rock formation that an electrical tool cannot see or can see only imperfectly. As will be explained later, where applicable, acoustic methods provide continuous circumferential imagery of the borehole sidewall whereas electrical methods can only provide discontinuous segmental coverage.
A typical acoustic televiewer circumferentially scans continuously the sidewall at 6 revolutions per second, acoustically illuminating the sidewall 250 times per scan to provide an equal number of acoustic measurement samples per revolution while being drawn up the borehole at a uniform rate of 10 feet per minute. The resulting image is a function of the acoustic reflectivity of the sidewall. The vertical resolution is about 0.3 inch. One such device is taught in U.S. Pat. No. 5,179,541, issued Jan. 12, 1993 to V. C. Weido, assigned to the assignee of this invention.
Resistivity tools, which are generically different from acoustic imagers, consist of a plurality of arrays of electrodes, usually 24-32 per array, that are mounted on several pads, usually four to twelve. The pads with the electrodes are pressed against the borehole sidewall by spring-loaded or hydraulically-actuated expandable arms. A potential difference is established between a common electrode on the tool and the respective electrodes of the arrays on the pads and observed in a desired measurement sampling sequence. Variations in the measured potential or the measured current as a function of depth are a measure of the electrical resistivity (or the inverse, the conductivity) of the formation. Variations in resistivity may be displayed to provide a visual image of the texture of a segment of the sidewall of the borehole. For example, see U.S. Pat. No. 4,468,623 issued Aug. 28, 1984 to S. C. Gianzero et al.
When a two-dimensional array of electrodes is used in an imaging tool, the vertical elements of the array must be referred to a common depth. The electrical measurements are made at timed intervals. Using the sampling interval as a time base and given a constant tool velocity, depth shifting would be a simple matter. But the velocity of the tool varies due to cable bounce (or "yo-yo" motion) as it is being drawn up the borehole. Therefore an independent depth measuring means is needed which is taught by U.S. Pat. No. 4,567,759, issued Feb. 4, 1986 to M. P. Ekstrom et al. which employs a z-axis accelerometer in combination with a conventional dip-angle sensing device (dipmeter).
U.S. Pat. No. 5,008,625 issued Apr. 16, 1991 to Min-Yi Chan teaches a method and apparatus for measuring in fine detail, the spontaneous potential (SP) around and along a borehole wall. The tool employs electrode arrays mounted on four or more radially-expansible pads much like the pad-mounted electrode arrays of the resistivity tool cited above. The electrodes measure the spontaneous potential (sometimes called the streaming potential), relative to a reference electrode at the surface. The spontaneous potential is caused by fluid movement from the formation into the drilling fluid. This tool is useful in highly resistive drilling muds where a resistivity tool is ineffective. However, the SP tool has certain disadvantages due to noise caused by electrode polarization and stray currents created by unwanted bimetallic junctions.
U.S. Pat. No. 4,692,908, issued Sep. 8, 1987 to M. K. Ekstrom et al. teaches use of an acoustic transducer for measuring the stand-off between an array of resistivity-measuring buttons on the tool and the borehole sidewall. That technique is also used to measure the thickness of the mud cake on the borehole wall.
All pad-mounted electrode arrays as above described, necessarily have a fixed finite circumferential width. Because of their fixed width, the angular coverage of each array around the sidewall is inversely proportional to the borehole diameter. Therefore, the visual imagery display of the borehole sidewall provided by the electrical tools is necessarily presented in the form of a series of strip segments equal in number to the number of pads. The segments are isolated from each other by gaps of missing data, the angular widths of which gaps depend upon the borehole diameter.
Borehole tools of the type described must be properly oriented so that the grain of the structural images can be related to the areal geology. It is well known in the art to equip the imaging tools with magnetometers for defining the axial alignment of the tool with respect to magnetic north and accelerometers for defining the alignment of the tool with respect to gravity. Alternatively, an inertial guidance system may be provided for tracking the direction of the drill bit in deviated holes.
In the past, it has been known to survey a borehole using different tools during separate imaging runs, particularly where the tools are generically different such as electrical tools versus acoustic tools. A purpose for using separate imaging tools is, of course, to use acoustic data to fill in the gaps between the segmental coverage of the electrical data. One reason for separate tool runs was due in part to the problems arising from use of measurement devices of different genera that are characterized by different scanning and data-sampling rates, incompatible power-supply needs and different dynamic ranges of the measured data values.
The difficulty that arises using physically separate runs is to correlate precisely the imagery resulting from the acoustic data with the imagery resulting from the electrical data with respect to azimuth, borehole inclination and depth, as obtained from the two or more independent runs. That difficulty arises because of the vagaries of different timing bases, different hoisting cables that have different stretch coefficients and other well-known variables. Although it is known to provide several different species of devices on a single sonde for measuring selected petrophysical characteristics, the devices were generically similar such as particle counters for neutron, gamma-ray and N.sup.16 studies. Similarly, resistivity, self-potential and induction loggers belong to the same genus. A third type of logger incudes velocity-measuring sondes of various types.
There is a need for a means to furnish acoustic imagery continuity across the gaps between the segmented images provided by electrical tools and to provide a single tool that combines the desirable features of both electrical and acoustic imaging techniques with a minimum distance of axial separation between the various imaging sensors.